In a variety of applications an optical signal is coupled between an optical fiber and a mating optical component. In many of these applications there is an optical power limit that limits the optical power that can be used both in and out of optical fiber. Above the power limit, the fiber is subject to, catastrophic destruction of the fiber core, or other optically-induced damage.
Tapered optical connectors are known for expanding guided fields propagating in optical fibers. Conventional tapers guide the light in the transition region by reflections at the air-glass interface along the outer surface of the expanding taper guide. Various conditions are placed on the rate of increase of diameter with axial distance. In particular, one condition often imposed is that the taper be gradual enough to ensure smooth evolution of the guided mode of the fiber into a larger guided mode, without the substantial excitation of spurious optical modes in the taper. Such tapers are described as being substantially “adiabatic”.
In some cases, especially in fiber power beam delivery systems or high power fiber laser systems, the fiber is inherently multimode, i.e. the fiber core diameter and core/cladding refractive indices are such that many guided modes propagate. In this case the term “spot size” is often used to describe the region of fiber (i.e. core+small overspill into cladding) where the light intensity is substantial and consists of a superposition of many optical modes of the fiber. In this case, if a taper is formed or bonded to a fiber end, it will be with the intention of gradually increasing the spot size formed by this mixture of modes, simultaneously decreasing the optical intensity so that it has fallen to a value unlikely to cause optical damage when the beam encounters the final glass/air interface. Taper lengths to be effective for gradually increasing the spot size are generally on the order of several cms, making such devices impractical in certain applications. Moreover, the mode of operation of tapered devices described above undesirably results in optical reflections which tend to distort the beam emerging from the air/glass interface which reduces the ability of the device to form a free space beam of high quality to the right of the final surface
What is needed is a new fiber intensity reducing device for increasing the optical power that can be reliably launched into and out of optical fibers.